In deflection systems for television, computer monitors and other cathode ray tube display devices, deflection coils are provided on horizontal and vertical axes for scanning an electron beam at different frequencies along horizontal lines that are successively spaced vertically to define a raster. The respective horizontal and vertical deflection coils or yokes are oriented at perpendicular axes orthogonal one with the other. However, it is common for some coupling to occur between the horizontal and vertical yokes. Although the horizontal scanning frequency is substantially higher than the vertical frequency, coupling between the horizontal and vertical deflection systems can cause problems which require that the unwanted coupling be taken into account in the deflection system design.
In a deflection yoke assembly having a horizontal saddle coil and vertical torroid construction, the horizontal flux path flows from the center of a top vertical winding half toward the respective ends of that winding. However, this horizontal flux flows in the opposite direction in the bottom vertical winding half. Thus if the changing horizontal flux induces a voltage in the vertical coil, for example having a polarity plus to minus from center to edges of the top vertical winding half, the voltage induced in the bottom vertical winding half is in the opposite direction. Stray capacitance of the vertical windings when combined with vertical winding inductance results in a high Q resonant circuit that can generate a significant voltage, when excited at the horizontal frequency, when resonant at the center of the vertical winding layer. Hence when the vertical winding is excited by horizontal retrace energy, ringing can result which extends well into active horizontal video time. The resulting horizontal rate perturbation of the vertical scan cause apparent raster brightness changes in the form of vertical bars. The resonance effects of the vertical windings are spoiled by a damping network which is connected between center taps of the layers in each winding half that are subject to the unwanted resonance. The damping network causes a current to flow through the damping network which loads the resonant vertical coil Q. Thus excitation by horizontal retrace energy is greatly reduced and raster brightness bars are eliminated. However, a significant unwanted horizontal frequency current is coupled through the entire vertical winding and the vertical scan current sensing resistor. This horizontal frequency excitation of the vertical coils is substantially independent of the orthogonality of the windings.
An exemplary vertical deflection coil is driven by a deflection amplifier responsive to a vertical ramp signal, and forms part of a negative feedback loop responsive to a voltage developed across a current sensing resistor connected in series with the vertical deflection coil. The horizontal frequency damping network forms a horizontal frequency current which is also coupled via the current sensing resistor. Thus, the vertical deflection amplifier receives feedback signals having components due to the vertical scanning current and the unwanted horizontal deflection current, coupled via the damping network and representing, for example a second derivative of the horizontal current. As a result of this mixture of feedback signal components, the vertical deflection amplifier is required to possess sufficient dynamic range to generate an output signal which is responsive to the feedback signal mixture without signal clipping, asymmetrical limiting, transient response distortion or slew rate limitation. The damping network and resultant horizontal rate current are needed, but can cause additional deflection amplifier power dissipation and vertical deflection signal distortion. Thus it would be advantageous to utilize the vertical damping network but eliminate the horizontal rate current component from the vertical deflection feedback signal that controls the vertical amplifier.